In a variety of systems, such as magnetic recording systems having motors for driving the capstan of a tape recording unit, is important to set the rotational frequency (speed) and phase of a motor rapidly and accurately. Due to the complex dynamics of motors and phase-lock loops, conventional motor control circuits have typically been designed to have a both a speed control loop and a separate phase control loop.
Conventional speed control loops typically derive an error voltage from the motor's rotational frequency, or from the motor's period of rotation. This error voltage is fed back and compared with a reference voltage so that a correcting voltage can be generated and applied to the motor.
In conventional speed control loops of the type employing frequency-to-voltage conversion, a constant quantity of electric charge is typically transferred per cycle, and this charge is integrated or averaged to obtain the error voltage. This type of circuit is capable of achieving low input frequency ripple at the output, but is characterized by low dynamic response (i.e., low frequency-to-voltage conversion speed).
Because period-to-voltage conversion can be accomplished more rapidly (i.e., during a single motor cycle), period-to-voltage conversion is often employed instead of frequency-to-voltage conversion in conventional systems. However, conventional speed control loops employing period-to-voltage conversion are not stable over a wide range of motor speeds. The reason for this instability is that, because rotational period is the inverse of rotational frequency, period-derived error voltage signals are reciprocally rather than linearly related to motor speed. Conventional period-to-voltage speed control loops thus include a strong nonlinearity (such as a reciprocal or division element) which causes difficulty in maintaining loop stability over a wide range of motor speeds. Thus, use of period-to-voltage speed control circuits has typically been limited to applications in which the motor has only a single speed, or a few, switch-selectable speeds. Where the motor has more than one switch-selectable operating speed, it has usually been necessary to switch between discrete sets of loop parameters in conventional systems each time a different motor speed is selected.
It has not been known until the present invention how to achieve highly accurate motor speed and phase control over a wide and continuous range of motor speed and phase, with high dynamic response, and with constant loop parameters.